For example, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, predetermined numbers of the unit cells and the separators are stacked together to form a fuel cell stack.
Normally, as a fuel gas supplied to the fuel cell, a hydrogen gas produced from a hydrocarbon based raw fuel by a reforming apparatus is used. In the reforming apparatus, after a reforming raw material gas is obtained from the hydrocarbon based raw fuel such as a fossil fuel, e.g., methane or LNG, the reforming raw material gas is subjected to steam reforming or partial oxidation reforming, autothermal reforming or the like to produce a reformed gas (fuel gas).
Normally, the city gas used as a raw fuel contains, in addition to methane (CH4), hydrocarbon of high carbon (C2+) such as ethane (C2H6), propane (C3H6), and butane (C4H10). In the case of using the hydrocarbon of high carbon as a fuel of a solid oxide fuel cell, hydrocarbon of C2+ should be removed by reforming. It is because the carbon may be precipitated in the fuel pipe or on the anode to degrade the cell performance undesirably. In this case, the water vapor needs to be supplied excessively.
In this regard, for example, Japanese Laid-Open Patent Publication No. 2003-229163 discloses a preliminary reformer. In a steam reforming method of the preliminary reformer, reforming catalyst is filled in the preliminary reformer. In the preliminary reformer, an exhaust air from a fuel cell is utilized as a required heating source to remove hydrocarbon of C2+ from a fuel.
In the conventional technique, the preliminary reformer is operated in the temperature range of about 300° C. to 600° C. The S/C (steam/carbon) ratio is adjusted to be in the range of 1.5 to 6.0. That is, since the operating temperature of the preliminary reformer is high, in order to avoid precipitation (coking) of carbon, the S/C ratio needs to be considerably high. Thus, it is necessary to supply water excessively for the reforming reaction. Therefore, the capacity of the water supply power source such as a water pump needs to be large. As a result, the load of the fuel cell is large uneconomically.